1. Field of the Invention
This invention relates to water-dispersible urethane resin coatings prepared from drying or semidrying oils and having acid values lower than those heretofore known in the prior art.
2. Description of the Prior Art
Alkyds containing vegetable oil fatty acids have long been important as vehicles in protective coatings for exterior use on wood. Improvement of certain of their properties, e.g., durability, mildew and alkali resistance, has been attempted by chemical modification of the basic vehicle, but the processing conditions are rather extensive, usually requiring temperatures of about 210.degree. C. for about 10 hours.
Another approach to improve properties has been to prepare from vegetable oils synthetic alkyd resins containing amide linkages derived from amino alcohols. Gast et al., U.S. Pat. No. 3,485,779 and Gast et al., J. Amer. Oil Chem. Soc. 46(7): 360-364 (1969) teach the preparation of coating compositions by the modification of vegetable oils to urethane polyesteramides. These were prepared by first converting the vegetable oils with excess diethanolamine to their corresponding N,N-bis(2)-hydroxyethyl fatty amides (HEFA) and glycerol. The isolated HEFA were then reacted with dibasic acids or anhydrides at 140.degree.-150.degree. C. for 6-24 hours to yield the hydroxyl-terminated polyesteramides, which were finally modified with diisocyanate. Schneider et al., J. Paint Technol. 44(575): 58-63 (1972) later demonstrated a feasible commercial method to prepare the urethane polyesteramides in which the isolation of the intermediate hydroxyamides is eliminated.
Recently, petrochemical shortages and new laws restricting atmospheric organic solvent emissions have stimulated interest in water-dispersible coating vehicles. A number of these systems have been developed, but the processing energy required was high and similar to that for the conventional alkyd systems; and except for latex emulsion systems, water dispersibility required a minimum of 1 milliequivalent of carboxyl per gram of resin. This translates into an acid value of about 56.